The present invention relates to a method for enhancing the proportion of dialkyldihalosilane in an alkylhalosilane mixture produced during the direct reaction of powdered silicon, alkylhalide and copper catalyst. More particularly, the present invention relates to the employment of a volatile promoter, for example, an organophosphine, such as a trialkylphosphine, to enhance the proportion of dialkyldihalosilane in an alkylhalosilane direct method mixture.
As shown by Rochow, in U.S. Pat. No. 2,380,995, a mixture of alkylhalosilanes can be obtained by the direct reaction between powdered silicon and an alkylhalide in the presence of a copper-silicon alloy, which is referred to hereinafter as the "direct method". Among the principal alkylhalosilanes formed by the direct method using methyl chloride, there are included methyltrichlorosilane, referred to hereinafter sometimes as "T", and dimethyldichlorosilane, referred to hereinafter sometimes as "D". D has the highest commercial interest, because it is the source material of choice, and methods for reducing the T/D ratio in direct method alkylhalosilane mixtures are of significant interest to the organosilicon industry.
Significant improvements in direct method alkylhalosilane product selectivity are provided in the method of Ward et al, U.S. Pat. No. 4,500,724, incorporated herein by reference, by utilizing a powdered silicon-copper-zinc-tin "contact mass" which has been found to favor the formation of dialkyldihalosilane over alkyltrihalosilane. As shown in the Ward patent, there can be used a fluid bed, stirred bed, or fixed bed reactor. The term "contact mass" as used hereinafter, means a particulated material comprising silicon and copper, which can include promoters, such as zinc and tin. The contact mass can form after contact with alkylhalide and reaction conditions have substantially stabilized at temperatures in the range of about 250.degree. C. to 350.degree. C. Alternatively, "pre contact mass" can be formed prior to contact or reaction with alkylhalide, and can be prepared by heating mixtures comprising particulated silicon and copper salts, such as copper halide and optionally in combination with other metallic promoters, at temperatures in the range of about 280.degree. C. to 400.degree. C.
Additional techniques have been used to improve the yields of dialkyldihalosilane during direct method practice. For example, a promoter such as phosphorous in the elemental or combined form can be used. The use of metal phosphides, such as copper phosphide, is shown by Halm et al in U.S. Pat. No. 4,762,940. Degan et al, U.S. Pat. No. 5,059,706, show that an effective amount of certain volatile phosphorous compounds, for example, phosphorous trichloride, can be used to enhance the proportion of dialkyldihalosilane in the resulting alkylhalosilane mixture. More recently, elemental phosphorous has been used in combination with zinc, as shown in Halm et al, U.S. Pat. No. 5,596,119, and in Margaria et al, U.S. Pat. No. 5,714,131, employing a crystalline phase of metallurgical silicon having dissolved phosphorous.
While the prior art shows that phosphorous, either in the elemental or combined form can be effective as a promoter in the direct method for enhancing the production of dialkyldihalosilane, additional techniques for introducing phosphorous as a promoter into the direct method are constantly being sought. As taught by Degan et al, U.S. Pat. No. 5,059,706, phosphorous trichloride, has an advantage over non-volatile phosphorous compounds, such as copper phosphide or elemental phosphorous dissolved in metallurgical silicon. For example, phosphorous trichloride does not require a metallurgical pretreatment or the intimate mixing of solids in order to provide a reduced amount of phosphorous into contact with a catalytically activated surface. Moreover, a volatile phosphorous compound is often easier to handle and can be more directly admixed with the gaseous stream of alkyl halide fed into the reactor.
Experience has shown, however, that while phosphorous trichloride has certain advantages over non-volatile phosphorous compounds as previously set forth, it is not widely used as a promoter in the production of dialkyldihalosilane by the direct method; phosphorous trichloride has been found to provide only a temporary improvement in the increase of % D when used as a phosphorous promoter in the direct method.